1. Field of the Invention
This invention relates to a chain-off forming apparatus for cover stitch sewing machines, and more particularly to a chain-off forming apparatus for cover stitch sewing machines, adapted to continuously form chain-off stitches knitted out by needle threads and a looper thread.
2. Description of the Related Art
In a conventional sewing machine for forming double chain stitches and cover stitches by a plurality of needles and one looper, the forming of chain-off stitches is required so as to cut and draw out a sewing thread by which subsequent fabrics are sewn after the sewing of preceding fabrics has been finished. For example, in a cover stitch sewing machine, such chain-off stitches are formed by knitting out needle threads and a looper thread by only the needles and a looper under a needle plate after the fabrics have been sent out and left a sewing position.
Such chain-off stitches drawn out by feed dogs are not engaged with any part until they have been held between an upper surface of the needle plate and a lower surface of a pressure foot. A chain-off stitch formed at a rear end of the fabrics is shaked off due to the vertical movements of the needle and the arcuate reciprocating movements of the looper. Consequently, a skip occurs, and the forming of continuous, normal chain-off stitches becomes impossible, so that there is a fear of entanglement of the threads in the looper and breakage of the threads.
In order to eliminate these difficulties, a needle plate and feed dogs for intermittent sewing work (Japanese Utility Model Publication No. 5653/1964), an apparatus for forming stitch chains by a two-needle type sewing machine (Japanese Patent Publication No. 58191/1982), a needle plate for hemstitch sewing machines (Japanese Utility Model Publication No. 25754/1986), a thread loop forming apparatus for multi-needle loop stitch sewing machines (Japanese Patent Laid-Open No. 23887/1991), an apparatus for forming double chain stitches, and feed dogs and a needle plate therefor (Japanese Patent Laid-Open No. 237275/1993), a method of and an apparatus for drawing out chain-off's by a two-needle and three-thread sewing machine, and a chain-off (Japanese Patent Laid-Open No. 300991/1993), a chain-off forming apparatus for cover stitch sewing machines (Japanese Patent Laid-Open No. 182079/1994), a double chain stitch sewing machine (Japanese Patent Laid-Open No. 8667/1995), a multi-needle double chain stitch sewing machine (Japanese Patent Laid-Open No. 275546/1995) and a thread control apparatus for double chain stitch sewing machines (Japanese Patent Laid-Open No. 285666/1997) were proposed.
(1) The needle plate and feed dogs for intermittent sewing work disclosed in Japanese Utility Model Publication No. 5653/1964 comprise two-group feed dogs, a lateral frame formed between holes made in the needle plate for the two-group feed dogs, and a pressure foot, so that chain-off's formed in a lock stitch sewing machine can be reliably retained.
(2) The apparatus for forming stitch chains by a two-needle type sewing machine disclosed in Japanese Patent Publication No. 58191/1982 is constructed so that, when the absence of fabrics is detected, the tension of the thread on one of the two needles is relieved to enable the tension-relieved needle thread to lie along a double chain stitch chain-off formed by the other non-tension-relieved needle thread. Accordingly, cover stitch chain-off's can be formed.
(3) The needle plate for hemstitch sewing machines disclosed in Japanese Utility Model Publication No. 25754/1986 is provided with chain-off guide grooves for guiding double chain stitch chain-off's in the fabric feeding direction, so that double chain stitch chain-off's can be formed.
(4) The thread loop forming apparatus for multi-needle loop stitch sewing machines disclosed in Japanese Patent Laid-Open No. 23887/1991 is provided with a sewing thread braking unit below a stitch forming member provided between two stitch hole slits of a stitch plate, and on the rear side of a sewing needle with respect to the sewing direction, so that the sewing thread braking unit can act as a substitute for non-existing knitted fabrics, and this enables chain-off's to be formed.
(5) In the apparatus for forming double chain stitches, and feed dogs and a needle plate therefor disclosed in Japanese Patent Laid-Open No. 237275/1993, the feed dogs provided in through holes, which are formed in a feed side portion of the needle plate, are divided in a position halfway between two needles, by which double chain stitch chain-off's are fed out, into front and rear groups with respect to the fabric feeding direction, and a stitch retaining web is formed at the portion of the needle plate which is between the two-group feed dogs, a web being also formed at the portion of the needle plate which is between a chain-off feed hole (thread passing slit) for a needle plate claw (U-shaped slit) formed thereon and the through holes in which the two-group feed dogs are provided.
This enables the chain-off stitch which has just been formed to be retained by the two-group feed dogs, two webs and pressure foot (fabric holder). Since a sensor for generating a fabric end signal is provided, a feeder driving mechanism becomes possible to increase a stitch length and reduce a feed rate on the basis of the fabric end signal. Therefore, continuous chain-off stitches can be formed.
(6) In the method of and apparatus for drawing out chain-off's by a two-needle and three-thread sewing machine, and a chain off, disclosed in Japanese Patent Laid-Open No. 300991/1993, an auxiliary blade is provided in parallel with a looper, and a free end portion of a needle plate claw (elongated plate) projects retractably from a needle hole of a needle plate. Rear chain-off feed dogs are provided on the rear side with respect to the fabric feeding direction, and guide grooves are formed so as to extend from a position on the left side of the needle hole toward the central portion of the rear feed dogs. The sewing machine is provided with sewing thread tension unit group and a chain-off forming thread tension unit group.
Owing to this arrangement, when the tension of the needle threads and looper thread is switched to low and high levels respectively at the time of completion of a fabric sewing operation, a slackened needle thread is brought close to the other needle thread and combined therewith into one string-like thread, which is guided in an auxiliary blade-supported condition toward the central portion of the rear feed dogs via the guide grooves and fed out by the rear feed dogs. Accordingly, stable chain-off's can be smoothly drawn out.
(7) The chain-off forming apparatus for cover stitch sewing machines disclosed in Japanese Patent Laid-Open No. 182079/1994 is provided with a means for detecting the presence or absence of fed fabrics, looper thread tensing means adapted to be operated in accordance with a detection result sent from the fabric detecting means, and increase the tension applied to the looper thread, a needle thread slackening means for collectively reducing the tension applied to each of the needle threads, a means for correcting a feed rate of the rear feed dogs to a higher level, and a chain-off thread holder provided across a needle plate so as to divide a hole, in which the feed dogs are exposed, into front and rear portions, and steadily holding chain-off's with a gripper.
This arrangement enables while the fabrics to be sewn are not fed, the tension of all the needle threads to be reduced to a level at which the tension is not gotten rid of and that of the looper thread to be increased at the same time, and stable chain-off's in which a plurality of slackened needle threads are tensed by the taut looper thread to be formed without causing the needle threads to be slackened.
(8) The double chain stitch sewing machine disclosed in Japanese Patent Laid-Open No. 8667/1995 is provided with a loop holder set on a needle guard and supporting needle thread loops, which are formed on needles, from the side portions thereof, a claw member extending from needle holes of a needle plate in the fabric feeding direction, projecting at one section of a front end region thereof in the fabric feeding direction to be enclosed with a stepped slit and thereby forming a projecting portion exclusive of a part of the section of the front end region thereof with the other end section thereof cut off, guide grooves formed in an upper surface of a lateral bar between the slit and feed dog holes so as to extend from the central portion of the lateral bar toward a rear side portion thereof, a means for switching an operating mode of a thread tension unit to a fabric sewing mode or a chain-off forming mode, and a thread take-up spring provided between the thread tension unit and an under-thread take-up means.
Since this arrangement enables the tension of the under-thread and a right needle thread to be switched to a high level and a low level respectively by a tension change-over means when the fabrics have finished being sewn, so that left and right needle threads are converged. Therefore, collective chain-off's can be formed, which can be held between rear feed dogs and pressure foot and drawn out via the guide grooves of the lateral bar. It is also possible to support needle thread loops laterally by the loop holder, and, moreover, an amount of movement of the needle thread hung on the looper in accordance with movement of the looper decreases owing to the claw member, so that the chain-off's can be formed and drawn out reliably.
(9) The multi-needle double chain stitch sewing machine disclosed in Japanese Patent Laid-Open No. 275546/1995 is provided with a U-shaped guide formed by making a pair of thread passing holes in a needle bar, and a thread spreader fixed to a sewing machine body and having a vertically extending cam, an operating surface of which is formed so as to bend the right needle thread, which is passed through a thread passing hole, gradually greatly when the U-shaped guide lowers in accordance with a downward movement of the needle bar. The relative positions of a thread path and a thread take-up cam are set changeable. A thread tension relieving means adapted to interrupt an operation with respect to threads of a second thread tension unit for looper thread during a sewing operation, and interrupt an operation with respect to threads of a first thread tension unit for right needle thread during the formation of chain-off's is further provided as a thread tension change-over means.
Owing to this arrangement, when the thread tension unit is switched at a terminal end of a sewing operation to form chain-off's, the flagging of the slightly slackened right needle thread is offset by the operation of the cam of the thread spreader, so that the formation of small loops and the protrusion thereof from chain-off's can be prevented, this enabling collected chain-off's to be drawn out. It is also possible to set a looper thread take-up rate of the thread take-up cam during a chain-off forming operation lower than that thereof during a sewing operation, thin collected chain-off's can be formed.
If the right needle thread and looper thread are passed through the respective thread tension units so as to relieve the tension of one of the thread tension units when the tension is switched, the occurrence of disorder of the threads can be prevented at the tension switching time.
(10) The thread control apparatus for double chain stitch sewing machines disclosed in Japanese Patent Laid-Open No. 285666/1997 is provided with a thread tension unit, which is adapted to apply tension to a looper thread, in the portion of a thread path which is between a looper and a thread take-up means for looper thread, and the thread tension unit has a means for interrupting the operation thereof only when chain-off's are formed. A needle guard is provided with a chain-off thread holder for supporting chain-off's from a position below a rear portion of the looper.
This arrangement enables the right needle thread slackened during the formation of chain-off's to be drawn out early and reliably, tight chain-off's to be formed, and the tension of the looper thread to be maintained in a suitably controlled condition, so that the chain-off's can be drawn out stably even when the sewing machine is operated at a high speed. Since the flagging of the needle threads and chain-off's in a position in the vicinity of a lower dead center of each needle can be prevented by the chain-off thread holder, the effect in preventing the breakage of the chain-off's ascribed to the double hanging of the thread on the looper can be improved.
In order to form chain-off stitches continuously in a stable condition in the inventions disclosed (1)-(10), it is necessary to detect the presence or absence of fabrics to be sewn, by a detecting means, such as a sensor, and vary the tension applied to needle threads and looper thread, by a thread tension control means in accordance with the result of the detection of the execution or non-execution of the feeding of fabrics to be sewn. Special devices, such as a chain-off thread holder, a loop holder and a thread spreader have to be provided for forming stable chain-off's, and this causes the chain-off forming structure to be complicated. Even when such special devices are provided, such chain-off's that meet only a certain narrow range of sewing conditions can be obtained, and it can hardly be said that such chain-off's meet a wide range of sewing conditions.